1. Field of the Invention
The invention relates in general to the apparatus and method for mobile unit positioning, and more particularly to the apparatus and method for mobile unit positioning utilizing angle of arrival and the timing information of the base station.
2. Description of the Related Art
The wireless communication system is popularly used for convenience. Because of mobile users' safety, the wireless communication system is recently required to be capable of determining the positions of mobile units. Because of the positions of mobile callers, the emergency responsiveness of the emergency service, such as 911 service, can be improved. A network-based position location system, for example, uses the given position of a base station (BS) to locate a mobile unit (MU) of the BS. The MU may be a cellular phone or other mobile communication device. The conventional positioning system determines the position of an MU by either the angle-of-arrival (AOA) of the incoming wireless signal or the timing-advance parameter thereof.
FIG. 1 is a diagram showing a mobile positioning system by using AOA. In FIG. 1, an MU is positioned at the intersection of the first line of bearing (LOB) L1 and the second LOB L2. The first base station, BS1, and the second base station, BS2, each have an antenna array. A wireless signal 110 is emitted by the MU in all directions and then arrives at the base stations BS1 and BS2. The AOA θ1 of the BS 1 and the AOA θ2 of the BS2 are obtained by performing an array signal processing on the received wireless signal. LOB L1 and LOB L2 are generated according to the angles of arrival θ1 and θ2, and then the MU is located at the intersection of LOB L1 and LOB L2. From the above description, at least two BSs are required to determine the position of the MU. For more accurate position of the MU, more BSs are required to estimate the AOA of the MU. However, the complexity of the base stations increases. In the existing wireless communication system, each base station using space diversity technology has two or three antennas only, while the above-mentioned base station with AOA positioning capability needs an antenna array. The base station using space diversity technology is widely used because it is simpler and able to reduce most of the fading effect. Thus, great efforts in modifying the existing wireless system makes the AOA mobile positioning system difficult to realize in practice. In addition, the cost is much higher if antenna-array base stations are used.
FIG. 2 is a diagram showing a mobile positioning system by using the single cell timing-advance (TA) positioning method with cell global identity (CGI). The cell of each base station has a specific CGI for identifying the position of the BS. The timing-advance parameter is the time that the wireless signal 110 takes to transmit from the MU to the BS. The distance D between the MU and the BS is obtained according to the speed of the wireless signal 110 and the timing-advance parameter. The MU is located at the circle around the BS with a radius of the distance D. The accuracy of the position of the MU depends mainly on the accuracy of the timing-advance parameter. In GSM system, the error of the timing-advance parameter is ½ bit, which results in a location estimation error of 550 meter. In other words, the position of the mobile unit in the GSM system lies on the ring with a width of 550 m around the base station. The position of the mobile unit can be more definite if more base stations are used in the mobile positioning.
The disadvantages of the conventional mobile positioning methods are as follows: (1) It is uneconomical and complicated in implementation. The AOA positioning method requires at least two base stations to determine a mobile location and each of them needs to be formed with an antenna array, for example. (2) It is cost-inefficient for obtaining a more accurate mobile location. In the TA positioning method, for example, a single base station is incapable of positioning accurately. For obtaining more accurate mobile location, multiple base stations should be required.